1. Field Of The Invention
This invention relates to safety joints used in downhole tool strings, and more particularly, to a hydraulic safety joint having a hydraulic pressure relief means for actuation of the safety joint.
2. Description Of The Prior Art
Safety joints are commonly used in the petroleum industry, the object being to provide a capability for retrieving as many stands of a pipe string as possible as well as any associated tools when a portion of the pipe string becomes stuck in a well bore, a not uncommon occurrence during drilling and open hole testing. In particular, during tubing conveyed perforation operations, the perforating guns sometimes become stuck in the well bore making it difficult or impossible to unset the retrievable packer being used in the operation. This situation requires a secondary operation to cut the work string below the packer and above the perforating guns so that the packer may be unset and the work string, excluding the perforating guns, removed from the well. When it is necessary to cut the work string in this manner, there is the additional cost for the recovery operation and the loss of time associated with it. In addition to this disadvantage, the recovery operation does not always leave an upper end on the perforating gun portion which will facilitate fishing operations which must be performed later to remove the guns. Thus, some sort of safety joint between the packer and the guns is desirable.
Two prior art safety joints of conventional design are the Halliburton Services VR safety joint and anchor pipe safety joint, disclosed in Halliburton Sales & Service Catalog No. 43, pages 2539-2540. The VR safety joint is operated by reciprocating the pipe string up and down while maintaining right-hand torque. Pipe string reciprocation and right-hand torque backs off a left-hand exterior threaded nut within the housing. The nut prevents the mandrel of the safety joint from coming free from the housing during normal pipe string movement.
The anchor pipe safety joint is operated by neutralizing the weight of the pipe string at the location of the safety joint and rotating the pipe string to the right. The rotation backs off a left-hand exterior threaded nut within the housing.
U.S. Pat. No. 4,246,964 to Brandell, assigned to the assignee of the present invention, discloses a safety joint using reciprocation of the pipe string in conjunction with a fairly low level of right-hand torque on the downstroke of the string. This safety joint is designed for use in situations where torque applied to the tool string is limited by the ability of some tools in the string to withstand the torque.
Another prior art safety joint is the Halliburton Services RTTS safety joint, which operates in much the same manner as the above-mentioned VR safety joint, utilizing right-hand torque and pipe string reciprocation. However, the RTTS safety joint includes a tension sleeve which must be parted by application of a predetermined tensile force on the pipe string before the tool can be operated by reciprocation. A problem with this type of safety joint is that the release tension may not be as precisely determined as desired in some cases. Further, to vary the release tension, the tool must be broken down and the tension sleeve replaced.
The present invention provides a hydraulic safety joint which utilizes a hydraulically ruptured disc as the initial release mechanism. The amount of tension required to actuate this release mechanism is predetermined by the pressure rating of the rupture disc. This feature allows the release tension of the tool to be easily adjusted with no modifications to the tool other than changing the rupture disc itself. Further, the rupture disc has a very accurate burst pressure which makes determination of the release tension more precise than with previously known devices such as the tension sleeve type safety joints.